In sensor networks, sensor localization is one of the mandatory functions for physical sensing. In this paper, we propose a sensor localization system that utilizes wireless local area network (WLAN) access points (APs) as anchor nodes. WLAN is widely available nowadays, and WLAN APs are used in many environments. Therefore, for our localization system, we utilize the WLAN APs already installed in the environment, thereby eliminating the installation of new anchors. We present an AP received signal strength (RSS) measurement method, which is the key technology of our sensor localization system. Notably, sensor nodes equipped with an IEEE 802.15.4 (ZigBee) module cannot detect WLAN AP signals. We therefore developed an AP signal detection method with AP identification by employing a cross-technology signal detection technique. Experimental evaluations demonstrate that our AP signal detection method successfully identified sender APs with a false positive (FP) detection rate less than 20%. The experiments also validated that the measured AP-RSS followed the same distribution as the RSS measured using a WLAN device.